Electrochemical Destruction of Insensitive High Explosives Using Magnéli Phase Titanium Oxide Reactive Electrochemical Membranes

Magnéli phase Ti4O7 reactive electrochemical membranes (REMs) were studied for the oxidation of insensitive high explosives, specifically 2,4-dinitroanisole (DNAN) and nitroguanidine (NQ). At a potential of 3.2 V/SHE for DNAN and 3.8 V/SHE for NQ, >99.7% removal of both compounds was achieved with a hydraulic residence time of ∼11 s. The pseudo-first-order rate constants were 38 min–1 for DNAN and 8.5 min–1 for NQ. Using 20 mM NQ and 0.62 mM DNAN as synthetic munitions wastewater, a three-stage reactor achieved 99.995% NQ removal, and ∼100% of DNAN was mineralized in a single stage. However, electrochemical oxidation of NQ caused electrode blocking and inhibition of NQ removal over time. Testing different background electrolytes revealed that in situ generated reactive chlorine species reacted with NQ and prevented electrode blocking, resulting in 97.2% removal during 3.5 h of continuous treatment. Based on experimental results and density functional theory simulations, it was concluded that the initial oxidation step for NQ was direct electron transfer (DET) and for DNAN was a combination of DET and reaction with OH•. Furthermore, cyanamide was likely the key intermediate during NQ oxidation that led to electrochemical polymerization reactions that blocked the electrode surface.